Light sources are used in a wide variety of applications. Continuous light sources produce light for an extended interval of time. Intermittent light sources produce a series of flashes. Spectral stability of the light source is an important property for many applications.
Metal halide lamps are a fairly new lamp technology that has many advantages, such as high efficiency. However, metal halide lamps have considerably less spectral stability than many other lamp technologies. This spectral instability severely impairs the usefulness of metal halide lamps in digital imaging scanners, such as photographic film input scanners.
In a digital film input scanner, the spectral transmission of the film is measured, pixel by pixel, and converted to digital data representing the image content of the film. Spectral transmission of the film is measured by illuminating the film with a light source, and measuring the light which passes through the film with a spectrally sensitive detector system. Many different types of light sources may be used, such as tungsten lamps, fluorescent lamps, xenon arc lamps, flash lamps, LEDs, lasers, and metal halide lamps. Many different types of detectors may be used, such as CCDs, photodiodes, photomultipliers, and CMOS sensor arrays. The spectral measurement of the light passing through the film may be accomplished in many ways, such as color filter arrays (CFAs) which make different sensor pixels have different spectral responses (such as red, green, and blue), alternating color filters (such as measuring first through a red filter at every pixel, then a green filter, then blue), alternating light source spectra (such as first turning on a red lamp only, then green only, then blue only) and tilted dichroic filters (such as to direct red light to one sensor array, green light to a second sensor array, blue to a third sensor array). Generally, unintended spectral instabilities in the light source, i.e. spectral noise, will be manifested as errors in the digital data produced by the scanner.
A "Method and Apparatus for Correcting Drift in the Level of a Video Signal in a Telecine" is disclosed in U.S. Pat. No. 5,173,780 granted to Mead on Dec. 22, 1992. Here, a CRT scanning means scans a film and performs an auxiliary scan through a film perforation. The auxiliary scan gives rise to a video signal which corresponds to a "no film" condition. The video signal from the auxiliary scan is fed to a comparator and then to a control unit which increments or decrements the photomultiplier output control.
A scanning method is disclosed in U.S. Pat. No. 5,406,070 to Edgar, entitled "Method and Apparatus for Scanning an Object and Correcting Image Data Using Concurrently Generated Illumination Data", granted on Apr. 11, 1995. The method includes the steps: using a first portion of a light sensor to generate image data of an object; using a second portion of a light sensor to generate illumination data of an illumination source concurrently with the generation of image data by the first portion of the light sensor; and correcting the generated image data using the concurrently generated illumination data.
An apparatus for scanning a photographic film, which has also a magnetic recording track, is disclosed in U.S. Pat. No. 5,692,094 to Tsukamoto, entitled "Film Image Scanning Apparatus for Picking up an Image and Magnetic Data Recorded on a Film Without Deterioration from Noise Generated By a Peripheral Circuit", granted Nov. 25, 1997. A light source illuminates a developed silver salt photographic film having a magnetic recording portion on which magnetic data is recorded. A light source drive section drives the light source. An image read section reads image data from the film illuminated by the light source. A reproduction section reproduces magnetic data recorded on the magnetic recording portion of the film. A power feeding control section prohibits a power feeding to the light source drive section or reduces an amount of power feeding when the magnetic data is reproduced by the reproduction section.
The above cited prior art does not tackle the problem of a light source or illumination system which is compensated for spectral fluctuations, so that devices with such illumination systems provide a more reliable output. U.S. Pat. No. 5,406,070 applies a system which allows only the control or the adjustment of intensity fluctuations of the lamp used in the light source. The film image scanning apparatus as disclosed in U.S. Pat. No. 5,692,094 is able to pick up image data and magnetic data recorded on the film. The apparatus does not provide a correction of the lamp according to spectral fluctuations. The power feeding to the lamp is reduced when magnetic data is reproduced from the magnetic layer.
According to the above there is no suggestion to provide an apparatus for the compensation of spectral fluctuations of a lamp used in a light source. It has to be assumed that the above cited prior art uses lamps or illumination systems which provide a fairly constant illumination with respect to spectral fluctuations. High output lamps and/or illumination systems with a constant spectral emission are expensive and cumbersome.